A conventional method for producing a molded metallic article involves melting the raw material and then casting the molten metal to form a shaped article. Typically, the cast article is subjected to a machining operation afterwards to give the final shape.
However, some materials such as intermetallics, ceramics and their composites referred to herein as high temperature materials melt at such high temperatures that conventional melting and casting methods are difficult or even impossible to use. Though conventional melting and casting methods are applicable to some intermetallic materials, often it is difficult to control the chemical composition of the cast material because of the large differences in melting temperatures and vapor pressures of the constituent elements of the intermetallic. For example, in the case of an intermetallic such as TiAl, the melting points of Titanium and Aluminum are 1668° C. and 660° C. respectively. Also the microstructure of the cast article may not be satisfactory because of the formation of large grains, dendrites and the chemical heterogeneity of the casting components. To obtain a desired microstructure of the cast article, post-casting operations are often required such as hot plastic deformation processes, including forging, rolling or extrusion.
Cast high temperature materials are extremely hard and are difficult to process by hot deformation. Accordingly, the manufacture of a shaped or formed article from these materials by melting, casting and machining is very difficult and expensive.
Powder metallurgy can be used to form sintered articles of high temperature materials. Use of powder metallurgy overcomes the problems caused by the chemical composition and lack of homogeneity of the constituents, but the products usually contain lots of voids. To densify the sintered products, long period hot pressing, or isostatic hot pressing, or heavily hot plastic deformation is required. This greatly increases the cost and production cycle of components.
Another alternative to a conventional casting and forging methods is a semi-solid forming process. Semi-solid forming is an advanced manufacturing method that is used to produce near net shape metal components in place of traditional casting and forging methods. In semisolid forming, a metal billet is brought to a thixotropic state and then formed to a desired shape, sometimes called a shaped component, by forging, extrusion or pressure die casting. The semi-solid forming can be a “rheoforming” or “rheocasting”, in which a semi-solid slurry is produced from liquid state under specific conditions, then directly delivered to a die for forming. Normally in this process 60 to 70% of the material is liquid. The semi-solid forming can also be a “thixoforming”, in which a specially prepared solid billet is heated to the semi-solid state, then delivered to and formed in a die. In this process, usually 30 to 40% of the material is liquid. A metal in a thixotropic state means that about 30 to 40% of the metal mass is in a liquid phase and the rest is in a solid phase and the solid phase comprises round shaped nodules suspended in the liquid phase.
In a thixotropic state, a metal exhibits unique rheological properties. In this respect the metal has a high viscosity at rest and a low viscosity when subjected to a high shear rate such as during a shaping process like forging or die casting. Accordingly, a thixotropic billet having a high viscosity at rest will retain its outer shape, therefore it can be easily handled and manipulated without substantial deformation. The low viscosity created by a shearing action then allows the billet to be shaped. This low viscosity property makes such a billet extremely suitable for a shaping process like forging or die casting.
Semisolid forming has many advantages over conventional casting. It allows lower operating temperatures and reduced-heat content which, in turn, results in reduced solidification shrinkage, less shrinkage porosity and longer life of the shaping die. Viscous flow of the semisolid materials during the filling of a die cavity results in low gas entrapment and low porosity of the resulting article. Semisolid forming has a relatively short process cycle so that automation of the process can result in high productivity. Precision dies may be used in the process so that the resulting article is close to a final shape requiring little machining. Compared to conventional forging, semisolid forming requires less shaping energy and the liquid-like flowability of the material allows the formation of complex shapes.
The current semi-solid forming technology has its drawbacks. These include the requirement for specially prepared starting material. The key to the semisolid forming is the round non-dendritic microstructure of the starting material. To obtain a suitable microstructure of the starting material, an additional process is required to break up the inherent dendritic microstructure of billets. For example, vigorous agitation processes, which were disclosed in U.S. Pat. No. 3,948,650, U.S. Pat. No. 3,954,455, U.S. Pat. No. 4,310,352 (mechanical stirring) and U.S. Pat. No. 4,229,210 (inductive electromagnetic stirring), were used to break up dendritic microstructure during billet casting.
Instead of vigorous agitation processes, a “SIMA” (strain induced, melt activated) process which was disclosed in U.S. Pat. No. 4,415,374 was used to make starting materials for semi-solid forming. In this process, a conventionally solidified and homogenized ingot was hot deformed, then cold deformed to obtain a directional grain structure. When the deformed material was reheated to a temperature above the solidus and below the liquidus, a semi-solid material with uniform discrete round particles and liquid matrix was formed.
A process called “nucleated casting” which was disclosed in U.S. Pat. No. 5,381,847 and U.S. Pat. No. 6,068,043 was also used to make semi-solid forming starting materials. In this process, metal liquid was sprayed in a non-reactive gas environment to form droplets. The partially solidified droplets were then collected to form a billet which was suitable for semi-solid forming.
An additional drawback of current semisolid forming technology is the high cost of starting materials because of the additional process required to form the round non-dendritic microstructure. Although the nature of semisolid forming makes it an ideal process for the manufacture of high melting temperature alloys such as superalloys, tool steels and intermetallic materials that are difficult to shape by conventional means, the technique currently is commercially limited to production involving only certain aluminum and magnesium alloys.
High melting temperature starting materials which are suitable for semi-solid forming might be produced requiring special and expensive capital equipment. For example, an induction-heating element with an accurate temperature control system is used to heat the billet to a semisolid state. In order to form the desired liquid fraction, the temperature of the billet often is required to be within ±5° C. or even ±2° C. of a desired value throughout the whole billet. This makes temperature control difficult and complicated. For high melting temperature materials, temperature control of the billet is even more difficult.
Another method for manufacture of high temperature materials involves combustion synthesis or self-propagating high-temperature synthesis (SHS). In combustion synthesis, reactant powders are mixed and pressed to form a compact. The compact then is ignited to trigger a combustion reaction. Highly exothermic reactions between reactant powders result in high temperature reaction products. Immediately after the exothermic reaction, the temperature of the resulting product is often equal to or higher than the melting temperature so the product of the reaction often is in a liquid or semisolid state.
One primary objective of the present invention is to manufacture high temperature material products by taking advantage of high rate of production, near-net shaping capability, low operation temperature of semi-solid forming techniques. Another objective is to take advantage of combustion synthesis techniques to bring a billet to a semi-solid state, therefore overcome the various above-mentioned limitations of semi-solid forming techniques.
The combustion synthesis technique and its prospects have been discussed in several recent review papers (1, Zuhair A. Munir and Umberto Anselmi-Tamburini, Materials Science Reports, Vol. 3(1989), p 279; 2, J. B. Holt and S. D. Dunmead, Annu. Rev. Mater. Sci., Vol. 21(1991), p 305; 3, John. J. Moore and H. J. Feng, Progress in Materials Science, Vol. 39(1995), p 243, 275; 4, A. Varma and A. S. Mukasyan, in ASM Handbook Volume 7: Metal Powder Technologies and Applications, W. B. Eisen, B. L. Ferguson, R. M. German, Ed., ASM International, 1998, p 523).
As compared to other methods of materials preparation, the advantages of combustion synthesis include: 1) a low energy requirement; 2) fast reaction rate and short reaction time; 3) relative simplicity of the process; and 4) easy introduction of reinforcements in the preparation of composites either by in situ formation of a second phase or by the addition of an inert second phase to the reactants. However, a major problem of combustion synthesis is that the resulting products are generally porous. Accordingly, combustion synthesis generally is limited to the production of powder materials. In order to obtain a dense material for components and structural applications by combustion synthesis, the products must be densified.
To this end various densification techniques are employed to eliminate the porosity of combustion synthesis products. Among them, pressure assisted densification is a most efficient and popular method. Several variations of the pressure assisted densification techniques have been developed. These include:                1) hot pressing or hot isostatic pressing during or immediately after combustion synthesis, as seen in U.S. Pat. Nos. 4,909,842, 4,946,643, 5,342,572, 5,382,405, 5,708,956, 5,794,113, 6,001,304, and JP 11131106;        2) high speed shock-wave pressing, as seen in U.S. Pat. No. 5,114,645, L. J. Kecskes, T. Kottke, A. J. Niiler, J. Am. Ceram. Soc., Vol 73(1990), p1274, B. H. Kabin, G. E. Korth, R. L. Williamson, Int. J. SHS, Vol 1(1992), p336; and        3) hot rolling or extrusion after combustion synthesis, as seen in U.S. Pat. No. 4,642,218, V. V. Podlesov, A. V. Radugin, A. M. Stolin, A. G. Merzhanov, J.Eng. Phys. Thermophys., Vol 63 (1992), p1065.        
All the techniques listed above have serious disadvantages. All methods are limited to producing simple shape products. A product having a shape which requires little or no subsequent machining is difficult to obtain by these methods except by hot pressing and hot isostatic pressing. However, hot pressing and hot isostatic pressing require long processing time and high processing temperature in order to obtain low porosity products. These processes generally require additional external heating to maintain reaction products at high temperatures. The requirements of long pressing time and additional heating detract from the advantages of combustion synthesis, these advantages being high speed and low energy requirement.
Accordingly, it is an object of the present invention to provide a method for manufacturing an article composed of high temperature materials that is sufficiently close to a desired shape as to require little or no subsequent machining.
Another object of the present invention is to provide a method for manufacturing articles of high temperature materials utilizing a semisolid forming process.
A further object of the present invention is to provide a method for manufacturing an article composed of a high temperature material wherein combustion synthesis is used to bring a billet to a semisolid state for forming to the desired shape.
Yet another object of the present invention is to provide an apparatus for forming an article composed of high temperature materials.